Monday, 10 January, 2011

Created as if by magic

The growth of silica nanochannels

Oriented nanochannels

Crystals are typical examples for high-order structures which are able to build up on their own. In a similar way also tiny silica channels can grow and coalesce. These structures play an important role for many applications of nanotechnology. The single channels have a diameter of only about 3 nanometers. Scientists of the Nanosystems Initiative Munich (NIM) have now developed a method, which allows watching the growth of these structures despite their diminutiveness (Nature Nanotechnoloy, Online January 10, 2011).

The work was carried out by members of the research groups led by Professors Christoph Bräuchle and Jens Michaelis at LMU’s Department of Chemistry. In their experiments the team employed fluorescent dye molecules to follow nanotube growth in thin films of mesoporous silica. The linear molecules diffuse into the channels and, with the aid of polarization fluorescence microscopy using a confocal laser scanning microscope, the orientation of single dye molecules within channels can be determined. This allows the researchers to pinpoint the orientation and monitor the growth of individual tubes. They then use atomic force microscopy to measure the areas of the ordered domains (in the x- and y-axes) and their degree of stacking (in the z-axis).

Using this set-up, it was possible to measure directly how long it took for channels to form and how fast they grew. Temperature and humidity were found to have a major effect on the growth parameters. When the reaction was allowed to proceed at 35°C, domains with sizes on the order of a few micrometers in length formed within minutes. Growth rates were lower at 25°C, but growth continued for hours, giving rise to much larger domains with dimensions of up to 0.3 mm.

The ability to monitor nanochannel growth in real time allowed the investigators to recognize two distinct phases in the process. Lamellar structures consisting of stacks of two-dimensional sheets of silica act as precursors for the generation of three-dimensional hexagonal arrays of silicate nanochannels. It turned out that this transformation occurs very rapidly every time a hexagonal channel domain impinges upon a pre-existing lamellar region.

Nanochannel structures as large as those detected in the new experiments are of great interest for numerous applications. They could, for instance, be used as implantable reservoirs for long-term drug delivery. Other possibilities include their use as molecular sieves for the fractionation of complex molecular mixtures and as chemical catalysts. Furthermore, confinement of particular types of molecules within nanochannels can give rise to special optical effects, which can be exploited in the field of nano-optics. (NIM)

Publication:

Visualization of the self-assembly of silica nanochannels reveals growth mechanism.